The present invention relates to protection against overheating of power electronic semiconductor switches connected in series with a load.
The IGBT is nowadays a natural choice for the designers of high power processing electronic circuits. However, the current tail of the IGBT, at turn-off, will introduce high switching losses and thus impose an unnecessary low limit of the power level that can be handled. A snubber circuit can be used as described by G. Hua, X. Yang, Y. Jiang, F. C. Lee, xe2x80x9cNovel zero-current-transition PWM convertersxe2x80x9d, IEEE Power Electronics Specialists"" Conference (PESC), June 1993. Another snubber circuit is described by Y. Jang, M. M. Jovanovi, xe2x80x9cA New, Soft-Switched, High-Power-Factor Boost Converter with IGBT""sxe2x80x9d IEEE International Telecommunication""s Energy Conference, 21st INTELEC, June 1999.
A straightxe2x80x94forward way of dealing with the current tail problem of the IGBT is to connect a MOSFET in parallel with the IGBT. The IGBT is used for reduction of the conduction losses and the MOSFET reduces the turn-off losses, in Y. M. Jiang, G. C. Hua, E. X. Yang, F. C. Lee, xe2x80x9cSoft-Switching of IGBT""s with the help of MOSFET""sxe2x80x9d, Proceedings of the Virginia Power Electronics Conference, Sep. 20-22, 1992, pp. 77-84. The disadvantage of this approach is that the output capacitance of the MOSFET increases switching losses at turn-on.
By using Zero-Voltage-Switching, ZVS,xe2x80x94at the time of turn-on, one is able to add capacitance to limit dv/dt at turn-off. An interesting comparative study was performed in K. Wang, F. C. Lee, G. Hua, D. Borojevic, xe2x80x9cA comparative study of switching losses of IGBT""s under hard-switching, zero-voltage switching and zero-current switchingxe2x80x9d, IEEE Power Electronics Specialists"" Conference (PESC), June 1994. A relatively large capacitor was introduced in parallel with the IGBT in order to limit the dv/dt of collector-emitter voltage at turn-off. Thus the current-voltage over-lap was reduced, which otherwise would have caused very high turn-off losses. Several 1200V/50A IGBTs were tested. The ZVS was compared to zero-current-switch ZCS-off. It turned out the ZVS-on concept was more efficient than the ZCS-off concept due to that by using ZVS-on one is able to add capacitance to limit dv/dt at turn-off.
A disadvantage may be that the optimum delay time varies between different types of the IGBT and vendors. Therefore, the delay needs to be long enough to include a margin for the worst case, (the longest current tail time). This disadvantage is not present to the same extent in the circuit presented in reference, xe2x80x9cA New, Soft-Switched, High-Power-Factor Boost Converter with IGBT""sxe2x80x9d where the switching of the IGBT can be performed at zero current. The disadvantage of this circuit is that the snubber""s MOS-FET""s VI-rating needs to be relatively large because the auxiliary switch in the snubber takes over the whole load current linearly during the on-time of the IGBT. This in turn is achieved by introducing a large circulating current in the snubber, which may deteriorate the overall efficiency.
Provided that the switching losses introduced by hard-turn-on of the IGBT are considered acceptable, it would still be necessary to reduce the losses introduced by the current tail if one wants to utilise the transistor to near its full capability.
Prior art for protection circuits are illustrated in FIG. 1-FIG. 4.
FIG. 1 shows a MOSFET S1 in parallel with the IGBT S. Upon turning off, the IGBT S is turned off first, and after a short delay the MOSFET S1 is turned off as well. In this way, the collector-emitter voltage of the IGBT will be kept low, while the current tail in the IGBT S is decaying to zero, so that the losses during this interval will be kept to a minimum. The disadvantage of this circuit is that the output capacitance of the MOSFET S1 will be discharged through the IGBT S, and/or the MOSFET S1 at turn-on.
FIG. 2 shows a snubber capacitor C1 in parallel with the IGBT S. The advantage and disadvantage are principally the same as in the described above with reference to FIG. 1.
In FIG. 3, a diode-capacitor clamping snubber is shown with a snubber diode D1 and a snubber capacitor C1. The snubber diode D1 is preventing the snubber capacitance C1 to discharge through the IGBT S. In order to reset the capacitor voltage to zero a reset circuit RC is needed as illustrated with a connection to a circuit symbolised by a box. The diode-capacitor clamping snubber as shown as D1, C1, would certainly reduce turn-off losses. However, the snubber capacitor C1 needs to be fully discharged before each turn-off of the IGBT.
In FIG. 4 a MOSFET S1 is connected in parallel with the IGBT S and a reset circuit RC is shown as an embodiment comprising a boost DC-to-DC-converter. The disadvantage with this circuit is the fact that the MOSFET S1 not only takes-over the current in the IGBT S through the diode D1, but also current flowing from the reset circuit RC connected to the drain terminal of the MOSFET S1 and cathode of the diode D1 causing a circulating current and additional conduction losses in the MOSFET S1.
It is an object of the present invention to overcome the problems as indicated above and to provide a zero-voltage-switching snubber circuit having improved characteristics.
This object is obtained by using an active switch, which keeps the collector-emitter voltage of the IGBT close to zero, so that the turn-off loss is minimised.
By introducing a circuit in parallel with the main switch (IGBT) the collector-emitter voltage can be clamped to almost zero during the time when the current tail is decaying to zero.
The circuit consists of an auxiliary switch, e.g. power MOSFET in parallel with a capacitor. The auxiliary switch and the capacitor are connected to the main switch via a diode. The diode prevents the capacitor from discharging through the main switch at turn-on.
A key feature of the invention is that circulating currents are minimised by a blocking diode, inter connected between an anode terminal of the blocking diode to the drain terminal of the auxiliary switch, and the cathode terminal of the blocking diode is connected to a reset circuit. The reset circuit is controlled in such a way, that it will discharge the output capacitor of the auxiliary switch while the main switch is conducting. After a delay, when the capacitor is fully discharged, the auxiliary switch is turned on under zero voltage condition.
An advantage of the present invention is that the gate drives for both switches are of the simple non-isolated type. Therefore there is no need for an extra control circuit; the same gate drive signal can be used for both switches with an addition of a simple delay of the gate drive signal of auxiliary switch. The auxiliary switch can be chosen with much smaller VI-rating than that of the main switch.
Another advantage of the present invention is that the proposed snubber does not introduce a need for higher voltage blocking capability of the transistors than what is required in an ordinary boost converter.